Separator for Electrochemical Device and Method for Manufacturing the Same

ABSTRACT

A separator for an electrochemical device is provided. The separator comprises a porous substrate having a plurality of pores and a porous coating layer positioned on at least one surface of the porous substrate, the porous coating layer including a plurality of inorganic particles and a binder polymer positioned on a whole or a part of the surface of the inorganic particles to connect the inorganic particles with one another and fix the inorganic particles, wherein the binder polymer comprises a first binder polymer and a second binder polymer. The first binder polymer is poly(vinylidene fluoride-co-hexafluoroproyplene) (PVdF-HFP), and the second binder polymer is poly(vinylidene fluoride-co-tetrafluoroethylene) (PVdF-TFE). The first binder polymer has an electrolyte uptake of 80-165%, and the second binder polymer has an electrolyte uptake of 20-40%. An electrochemical device including the separator is also disclosed.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a national phase entry under 35 U.S.C. § 371of International Application No. PCT/KR2019/011947 filed Sep. 16, 2019which claims priority from Korean Patent Application No. 10-2018-0109238filed on Sep. 12, 2018 in the Republic of Korea, the disclosures ofwhich are incorporated herein by reference in its entirety.

TECHNICAL FIELD

The present disclosure relates to a separator for an electrochemicaldevice having improved adhesion to an electrode, and a method formanufacturing the same. Particularly, the present disclosure relates toa separator for an electrochemical device which shows improved adhesionto an electrode and adhesion to a porous substrate and satisfies boththe resistance and air permeability required for a separator for anelectrochemical device, and a method for manufacturing the same.

BACKGROUND ART

Recently, a great attention has been given to electrochemical devices,particularly in terms of ensuring safety, in the field ofelectrochemical devices. Particularly, a secondary battery, such as alithium secondary battery, has an electrode assembly including apositive electrode, a negative electrode and a separator. Such anelectrode assembly may have a structure in which the separator isinterposed between the positive electrode and the negative electrode.

A separator used for a lithium secondary battery takes the form of aporous woven fabric or non-woven fabric, or is a porous separator havingpores formed through a dry process or wet process, in the case of a filmor membrane. However, such a porous separator uses a binder for itsbinding with an electrode, wherein the binder not only is coated on thesurface of a porous polymer substrate but also infiltrates into thepores of the porous polymer substrate, and thus causes a problem ofdegradation of the function of the separator as an ion channel.

More particularly, a stacked lithium secondary battery or stacked-foldedlithium secondary battery is manufactured with ease, has a structurewhich may utilize the space efficiently, and can maximize the content ofelectrode active materials to realize a highly integrated battery.

Particularly, a binder is used in order to bind a separator with anelectrode in a stacked-folded structure, and binders having a T_(m) of140° C. or lower are preferred for this purpose. When a separator iscoated, a process for forming pores in the coated binder through phaseseparation is used. However, when the phase separation shows excessivelyhigh kinetics, there is a problem in that most of the binder forms largepores on the coating layer and adhesion between the substrate and thecoating layer is low. On the other hand, when the phase separation showsexcessively low kinetics, there is a problem in that most of the binderis formed at the bottom surface of the coating layer with small pores orwith no porous structure, and no adhesive layer with the electrode isformed.

DISCLOSURE Technical Problem

The present disclosure is designed to solve the problems of the relatedart, and therefore the present disclosure is directed to providing aseparator for an electrochemical device which shows improved adhesion toa porous substrate and satisfies both the resistance and airpermeability required for a separator for an electrochemical device, anda method for manufacturing the same.

The present disclosure is also directed to providing a lithium secondarybattery including the separator having improved adhesion to anelectrode.

Technical Solution

In one aspect of the present disclosure, there is provided a separatorfor an electrochemical device as defined in any one of the followingembodiments.

According to the first embodiment of the present disclosure, there isprovided a separator for an electrochemical device which comprises:

a porous substrate having a plurality of pores; and

a porous coating layer positioned on at least one surface of the poroussubstrate, and including a plurality of inorganic particles and a binderpolymer positioned on the whole or a part of the surface of theinorganic particles to connect the inorganic particles with one anotherand fix them,

wherein the binder polymer comprises a first binder polymer and a secondbinder polymer,

the first binder polymer is poly(vinylidenefluoride-co-hexafluoroproyplene) (PVdF-HFP), and the second binderpolymer is poly(vinylidene fluoride-co-tetrafluoroethylene) (PVdF-TFE),

the first binder polymer has an electrolyte uptake of 80-165%, and

the second binder polymer has an electrolyte uptake of 20-40%.

According to the second embodiment of the present disclosure, there isprovided the separator for an electrochemical device as defined in thefirst embodiment, wherein the first binder polymer may have a meltingpoint of 130-135° C., and the second binder polymer may have a meltingpoint of 135-140° C.

According to the third embodiment of the present disclosure, there isprovided the separator for an electrochemical device as defined in thefirst or the second embodiment, wherein the first binder polymer mayhave a viscosity of 50-70 cp,

the second binder polymer may have a viscosity of 600-800 cp, and

the viscosity may be a viscosity value determined at a shear rate of100/s from slurry including 35 parts by weight of the first binderpolymer or the second binder polymer, 65 parts by weight of Al₂O₃ havingan average particle diameter (D₅₀) of 500 nm, and 400 parts by weight ofN-methyl-2-pyrrolidone (NMP).

According to the fourth embodiment of the present disclosure, there isprovided the separator for an electrochemical device as defined in anyone of the first to the third embodiments, wherein the first binderpolymer may have an electrolyte uptake of 90-130%, and

the second binder polymer may have an electrolyte uptake of 25-35%.

According to the fifth embodiment of the present disclosure, there isprovided the separator for an electrochemical device as defined in anyone of the first to the fourth embodiments, wherein the weight ratio ofthe first binder polymer to the second binder polymer may be50:50-85:15.

According to the sixth embodiment of the present disclosure, there isprovided the separator for an electrochemical device as defined in anyone of the first to the fifth embodiments, wherein the surface of theporous coating layer may include a plurality of nodes including theinorganic particles and the binder polymer covering at least a part ofthe surface of the inorganic particles, and at least one filament formedfrom the binder polymer of the nodes in a thread-like shape, and thefilament may include a node-linking portion extended from the node andconnecting the node with another node.

According to the seventh embodiment of the present disclosure, there isprovided the separator for an electrochemical device as defined in anyone of the first to the sixth embodiments, wherein the plurality ofnodes including the inorganic particles and the binder polymer coveringat least a part of the surface of the inorganic particles may forminterstitial volumes, while being in close contact with one another, atthe inner part of the porous coating layer, and the pores formed by theinterstitial volumes converted into vacant spaces totally may have asize smaller than the diameter of the inorganic particles.

In another aspect of the present disclosure, there is also provided amethod for manufacturing a separator for an electrochemical device asdefined in any one of the following embodiments.

According to the eighth embodiment of the present disclosure, there isprovided a method for manufacturing a separator for an electrochemicaldevice, including the steps of:

(S1) preparing slurry for a porous coating layer comprising a solvent,inorganic particles and a binder polymer, wherein the binder polymercomprises a first binder polymer and a second binder polymer;

(S2) applying the slurry for a porous coating layer to at least onesurface of a porous substrate; and

(S3) dipping the resultant product of step (S2) in a solidifyingsolution including a non-solvent,

wherein the first binder polymer is poly(vinylidenefluoride-co-hexafluoroproyplene) (PVdF-HFP), and the second binderpolymer is poly(vinylidene fluoride-co-tetrafluoroethylene) (PVdF-TFE),

the first binder polymer has an electrolyte uptake of 80-165%,

the second binder polymer has an electrolyte uptake of 20-40%, and

the slurry for a porous coating layer may have a viscosity value of100-400 cp, as determined at a shear rate of 100/s.

According to the ninth embodiment of the present disclosure, there isprovided the method for manufacturing a separator for an electrochemicaldevice as defined in the eighth embodiment, wherein the first binderpolymer may have a melting point of 130-135° C., and the second binderpolymer may have a melting point of 135-140° C.

According to the tenth embodiment of the present disclosure, there isprovided the method for manufacturing a separator for an electrochemicaldevice as defined in the eighth or the ninth embodiment, wherein thefirst binder polymer may have a viscosity of 50-70 cp,

the second binder polymer may have a viscosity of 600-800 cp, and

the viscosity may be a viscosity value determined at a shear rate of100/s from slurry containing 35 parts by weight of the first binderpolymer or the second binder polymer, 65 parts by weight of Al₂O₃ havingan average particle diameter (D₅₀) of 500 nm, and 400 parts by weight ofN-methyl-2-pyrrolidone (NMP).

According to the eleventh embodiment of the present disclosure, there isprovided the method for manufacturing a separator for an electrochemicaldevice as defined in any one of the eighth to the tenth embodiments,wherein the first binder polymer may have an electrolyte uptake of90-130%, and

the second binder polymer may have an electrolyte uptake of 25-35%.

According to the twelfth embodiment of the present disclosure, there isprovided the method for manufacturing a separator for an electrochemicaldevice as defined in any one of the eighth to the eleventh embodiments,wherein the weight ratio of the first binder polymer to the secondbinder polymer may be 50:50-85:15.

In still another aspect of the present disclosure, there is provided anelectrochemical device as defined in any one of the followingembodiments.

According to the thirteenth embodiment of the present disclosure, thereis provided an electrochemical device comprising a positive electrode, anegative electrode and a separator interposed between the positiveelectrode and the negative electrode, wherein the separator may be theseparator for an electrochemical device as defined in any one of thefirst to the seventh embodiments.

According to the fourteenth embodiment of the present disclosure, thereis provided the electrochemical device as defined in the thirteenthembodiment, which is a secondary battery.

Advantageous Effects

According to an embodiment of the present disclosure, a combination ofPVdF-HFP with PVdF-TFE is used for a porous coating layer, as acombination of two different types of binder polymers having a specificrange of electrolyte uptake different from each other. Therefore, theadvantages of the two types of binder polymers realize a synergiceffect, and thus it is possible to provide a separator for anelectrochemical device having improved adhesion to an electrode andadhesion to a porous substrate, and satisfying low resistance and highair permeability required for a separator for an electrochemical device,and a method for manufacturing the same.

DESCRIPTION OF DRAWINGS

The accompanying drawings illustrate a preferred embodiment of thepresent disclosure and together with the foregoing disclosure, serve toprovide further understanding of the technical features of the presentdisclosure, and thus, the present disclosure is not construed as beinglimited to the drawing.

FIG. 1 is an electron microscopic image illustrating the cross sectionof the separator according to Example 1.

FIG. 2 is an electron microscopic image illustrating the cross sectionof the separator according to Comparative Example 1.

DETAILED DESCRIPTION

Hereinafter, preferred embodiments of the present disclosure will bedescribed in detail with reference to the accompanying drawings. Priorto the description, it should be understood that the terms used in thespecification and the appended claims should not be construed as limitedto general and dictionary meanings, but interpreted based on themeanings and concepts corresponding to technical aspects of the presentdisclosure on the basis of the principle that the inventor is allowed todefine terms appropriately for the best explanation. Therefore, thedescription proposed herein is just a preferable example for the purposeof illustrations only, not intended to limit the scope of thedisclosure, so it should be understood that other equivalents andmodifications could be made thereto without departing from the scope ofthe disclosure.

In one aspect of the present disclosure, there is provided a separatorfor an electrochemical device which comprises:

a porous substrate having a plurality of pores; and

a porous coating layer positioned on at least one surface of the poroussubstrate, and including a plurality of inorganic particles and a binderpolymer positioned on the whole or a part of the surface of theinorganic particles to connect the inorganic particles with one anotherand fix them,

wherein the binder polymer comprises a first binder polymer and a secondbinder polymer,

the first binder polymer is poly(vinylidenefluoride-co-hexafluoroproyplene) (PVdF-HFP), and the second binderpolymer is poly(vinylidene fluoride-co-tetrafluoroethylene) (PVdF-TFE),

the first binder polymer has an electrolyte uptake of 80-165%, and

the second binder polymer has an electrolyte uptake of 20-40%.

The porous polymer substrate used for the separator according to thepresent disclosure may be any planar porous polymer substrate usedcurrently for electrochemical devices, and particular examples thereofinclude porous membranes or non-woven fabrics formed of variouspolymers. For example, a polyolefin-based porous membrane or a non-wovenfabric formed of polyethylene terephthalate fibers used as a separatorfor electrochemical devices, particularly a lithium secondary battery,may be used, and the materials and types thereof may be selecteddiversely depending on purposes. For example, the polyolefin-basedporous membrane may be formed of any one polyolefin-based polymerselected from the group consisting of: polyethylene, such as highdensity polyethylene, linear low density polyethylene, low densitypolyethylene and ultrahigh-molecular weight polyethylene; polypropylene;polybutylene; and polypentene, or a polymer blend containing at leasttwo of them. In addition, the non-woven fabric may be made of apolyolefin-based polymer or a polymer having higher heat resistance ascompared to polyolefin-based polymers.

The porous polymer substrate may have a thickness of 1-30 μm. Forexample, the porous polymer substrate may have a thickness of 1-20 μm,or 5-20 μm. When the porous polymer substrate has a thickness less than1 μm, it is difficult to retain physical properties. When the porouspolymer substrate has a thickness larger than 30 μm, battery resistancemay be increased.

The porous polymer substrate may have a porosity of 30-75%. For example,the porous polymer substrate may have a porosity of 35-65%. When theporosity satisfies the above-defined range, it is possible to prevent anincrease in battery resistance and to retain the mechanical propertiesof a porous polymer substrate.

The porous polymer substrate may have a pore size of 0.01-5.0 μm. Forexample, the porous polymer substrate may have a pore size of 0.1-1.0μm. When the pore size satisfies the above-defined range, it is possibleto prevent an increase in battery resistance caused by a blocked porestructure, and to retain self-discharge characteristics in a generallithium secondary battery.

Polyvinylidene fluoride (PVdF) homopolymer includes vinylidene-derivedrepeating units (—CH₂—CF₂—) with no comonomer-derived repeating units,and thus has a regular binding structure and relatively highcrystallinity. Therefore, it has the disadvantages of low solubility toorganic solvents and low electrolyte uptake.

One of the methods for overcoming such disadvantages is copolymerizationwith a comonomer.

PVdF copolymer obtained by introducing repeating units derived fromanother comonomer to PVdF homopolymer includes a portion having anirregular binding structure as compared to PVdF homopolymer and hasdecreased crystallinity. As a result, PVdF copolymer may have relativelyincreased electrolyte uptake.

Particularly, the electrolyte uptake of PVdF copolymer may be affectedby the structure, content and functional group characteristics of thecomonomer-derived repeating units introduced to PVdF homopolymer.

According to the present disclosure, the binder polymer includes amixture containing the first binder polymer and the second binderpolymer, wherein the first binder polymer is poly(vinylidenefluoride-co-hexafluoroproyplene) (PVdF-HFP), and the second binderpolymer is poly(vinylidene fluoride-co-tetrafluoroethylene) (PVdF-TFE).

Both PVdF-HFP and PVdF-TFE have a structure including HFT- andTFE-derived repeating units, respectively, introduced to PVdFhomopolymer. Herein, HFP has one more carbon atoms than TFE and its —CF₃group functions as a branch so that PVdF-HFP may have lowercrystallinity than PVdF-TFE.

However, even when using two types of binder polymers merely havinghigher electrolyte uptake than PVdF homopolymer for the porous coatinglayer, or using PVdF-HFP in combination with PVdF-TFE, it is notpossible to improve all of the characteristics required for a separator.

It is possible to realize improvement in all of the characteristics of aseparator, including resistance characteristics, air permeabilitycharacteristics, adhesion to a substrate, adhesion to an electrode and adensely packed coating layer structure, only when using PVdF-HFP havingan electrolyte uptake of 80-165% as the first binder polymer andPVdF-TFE having an electrolyte uptake of 20-40% as the second binderpolymer according to the present disclosure. This will be explained inmore detail hereinafter.

The first binder polymer, PVdF-HFP, has an electrolyte uptake of80-165%. According to an embodiment of the present disclosure, the firstbinder polymer may have an electrolyte uptake of 90-130%, 100-120%,105-120%, or 105-115%. When the first binder polymer satisfies theabove-defined range of electrolyte uptake, an electrolyte can betransported inside the binder to improve the affinity between the binderand the electrolyte, thereby providing a battery having improvedcharacteristics.

In addition, the second binder polymer, PVdF-TFE, has an electrolyteuptake of 20-40%. According to an embodiment of the present disclosure,the second binder polymer may have an electrolyte uptake of 25-35%,28-35%, or 28-33%. When the second binder polymer satisfies theabove-defined range of electrolyte uptake, the binder polymer is lessswelled, since an electrolyte is not absorbed thereto, and thus it ispossible to prevent pore-blocking of the porous polymer substrate causedby the binder polymer and to retain the porous structure. As a result,it is possible to provide low resistance and improved battery output.

According to an embodiment of the present disclosure, two types ofbinder polymers each having a different specific range of electrolyteuptake, i.e. poly(vinylidene fluoride-co-hexafluoroproyplene) (PVdF-HFP)having an electrolyte uptake of 80-165% as the first binder polymer, andpoly(vinylidene fluoride-co-tetrafluoroethylene) (PVdF-TFE) having anelectrolyte uptake of 20-40% as the second binder polymer are used incombination as a binder polymer. Thus, the advantages of each type ofbinder polymers realize a synergic effect. Therefore, it is possible toprovide a separator for an electrochemical device which has improvedadhesion to an electrode and adhesion to a porous substrate, andsatisfies low resistance, excellent air permeability and a denselypacked coating layer structure required for a separator for anelectrochemical device.

In addition, PVdF-HFP as the first binder polymer has —CF₃ branches inits polymer chain, and thus shows relatively low crystallinity ascompared to not only PVdF homopolymer having no comonomer-derivedrepeating units but also the second binder polymer, PVdF-TFE having nosuch branches. Therefore, PVdF-HFP facilitates electrolyte uptake andinfiltration of a non-solvent (e.g. water) into the polymer chain. Thus,PVdF-HFP as the first binder polymer may show a significantly high phaseseparation rate, when it is in contact with a non-solvent in a solutionstate. As a result, when slurry for forming a porous coating layerprepared by dispersing PVdF-HFP as the first binder polymer and PVdF-TFEas the second binder polymer in a solvent and dispersing inorganicparticles therein is coated on a porous polymer substrate, and then theresultant product is dipped in a non-solvent, PVdF-HFP as the firstbinder polymer contained in the slurry moves toward the coating layersurface that is in contact with the non-solvent at a significantlyhigher rate as compared to PVdF-TFE. Ultimately, PVdF-HFP as the firstbinder polymer may be present at a higher proportion (a higher contentof PVdF-HFP as compared to the weight ratio of PVdF-HFP to PVdF-TFE uponthe initial introduction) at the surface portion of the porous coatinglayer. On the contrary, PVdF-TFE as the second binder polymer has nobranches, unlike PVdF-HFP, shows a significantly lower infiltration rateof electrolyte and non-solvent into its polymer chain, and thus providesa relatively lower phase separation rate. Therefore, PVdF-TFE may bepresent at a higher proportion (a higher content of PVdF-TFE as comparedto the weight ratio of PVdF-HFP to PVdF-TFE upon the initialintroduction) at the inner part of the porous coating layer, i.e.between the porous polymer substrate and the inorganic particles.

When using the first binder polymer, PVdF-HFP, alone as a binderpolymer, it shows an excessively high phase separation rate so that thebinder polymer may be abundant at the surface portion of the porouscoating layer, resulting in significant degradation of adhesion betweenthe porous coating layer and the porous polymer substrate. On thecontrary, when using the second binder polymer, PVdF-TFE, alone as abinder polymer, it shows an excessively low phase separation rate sothat the binder polymer may be abundant at the inner part of the porouscoating layer. In this case, adhesion between the porous polymersubstrate and the inorganic particles may be improved, but adhesion toan electrode may be decreased significantly.

According to the present disclosure, PVdF-HFP as the first binderpolymer and PVdF-TFE as the second binder polymer are used incombination as a binder polymer so that PVdF-TFE may contribute toimprovement of adhesion to a substrate, i.e. adhesion between the porouspolymer substrate and the porous coating layer, and PVdF-HFP maycontribute to improvement of adhesion to an electrode at the surfaceportion of the porous coating layer. As a result, it is possible toprovide a separator having excellent adhesion to an electrode as well asexcellent adhesion to a substrate.

In addition, PVdF-TFE as the second binder polymer absorbs noelectrolyte, and thus the binder polymer is less swelled to prevent poreblocking of the porous polymer substrate, caused by the binder polymer.Further, it is possible to retain the porous structure at the inner partof the porous coating layer, thereby providing improved air permeabilitycharacteristics. While the inner part of the porous coating layerretains a porous structure, PVdF-HFP as the first binder polymer havinga higher electrolyte uptake as compared to the second binder polymer isabundant on the surface of the porous coating layer. Thus, the surfaceof the porous coating layer absorbs an electrolyte sufficiently tofacilitate transport of the electrolyte conducting lithium ions towardthe inner part of the binder polymer and porous polymer substrate of theseparator. In this manner, it is possible to improve batterycharacteristics, to reduce resistance, and to improve adhesion to anelectrode. The above-mentioned synergic effect derived from the combineduse of PVdF-HFP having an electrolyte uptake of 80-165% as the firstbinder polymer and PVdF-TFE having an electrolyte uptake of 20-40% asthe second binder polymer can be determined from the following Table 1.

Herein, the electrolyte uptake of PVdF-HFP and that of PVdF-TFE may bemodified by adequately controlling the content of HFP-derived repeatingunits, content of TFE-derived repeating units, weight average molecularweight of PVdF-HFP, weight average molecular weight of PVdF-TFE, or thelike.

The electrolyte uptake of the binder polymer (particularly, the firstbinder polymer and the second binder polymer) may be determined asfollows.

First, a film is prepared by using the binder polymer through a phaseseparation process, is cut into a circular shaped film (diameter 1.8 cm;area 2.54 cm²; thickness 16 μm (deviation, −0.5 to +0.5 μm)), and thenis weighed. Particularly, the film is obtained through dipping phaseseparation of a solution in which each binder polymer is dissolved byusing a non-solvent. Next, the circular shaped binder polymer film isintroduced to a mixed electrolyte solution (EC/DEC=1/1 or EC/PC=1/1,volume ratio) containing 1.0M of lithium salt (LiPF₆) dissolved thereinand is removed therefrom after 24 hours. Then, the binder polymer filmis allowed to stand in its vertically standing state for 1 minute toremove the liquid droplets on the film surface. The resultant filmimpregnated with the mixed electrolyte solution is weighed. The wholetest is carried out in a glove box filled with argon gas under acondition containing water less than 1.0 ppm. The electrolyte uptake (%)is calculated by the formula of [(Weight of film after uptake−Weight offilm before uptake)/(Weight of film before uptake)]×100.

According to an embodiment of the present disclosure, the first binderpolymer may have a viscosity of 50-70 cp, particularly 55-70 cp, andmore particularly 55-65 cp. When the above-defined range of viscosity issatisfied, it is possible to accelerate diffusion of the non-solvent andto increase the phase separation rate advisably.

According to an embodiment of the present disclosure, the first binderpolymer may have a melting point of 130-135° C., 130-133° C., or130-132° C. When the above-defined range of melting point is satisfied,it is possible to improve the stability, particularly heat stability, ofa separator.

Meanwhile, the second binder polymer may have a viscosity of 600-800 cp,particularly 600-750 cp, and more particularly 650-750 cp. When theabove-defined range of viscosity is satisfied, it is possible to slowerphase separation and to form uniform pores advisably. In general,viscosity may function as steric hindrance and affect the quenching rate(solidifying and film-forming rate) of a separator.

According to an embodiment of the present disclosure, the second binderpolymer may have a melting point of 135-140° C., 135-138° C., or135-137° C. When the above-defined range of melting point is satisfied,it is possible to improve the stability, particularly heat stability, ofa separator.

Herein, the viscosity of the first binder polymer means the viscosityvalue determined by mixing 35 parts by weight of the first binderpolymer, 65 parts by weight of Al₂O₃ having an average particle diameter(D₅₀) of 500 nm, and 400 parts by weight of N-methyl-2-pyrrolidone (NMP)by using a paint shaker at room temperature for about 90 minutes or moreto obtain slurry, and measuring the viscosity of the resultant slurry ata shear rate of 100/s. In addition, the viscosity of the second binderpolymer means the viscosity value determined by mixing 35 parts byweight of the second binder polymer, 65 parts by weight of Al₂O₃ havingan average particle diameter (D₅₀) of 500 nm, and 400 parts by weight ofN-methyl-2-pyrrolidone (NMP) by using a paint shaker at room temperaturefor about 90 minutes or more to obtain slurry, and measuring theviscosity of the resultant slurry at a shear rate of 100/s. Herein, thepaint shaker is a twin-arm paint shaker (multi-dimensional mixingmotion) favorable to high-viscosity slurry mixing and dispersibilityimprovement. The reason why such slurry containing Al₂O₃ having anaverage particle diameter (D₅₀) of 500 nm as inorganic particles is usedfor the determination of the viscosity of the first binder polymer andthat of the second binder polymer is that the first binder polymer andthe second binder polymer are used for preparing the slurry for a porouscoating layer of a separator subsequently, and thus the viscosity ofeach binder polymer in a slurry state containing inorganic particles isexamined in advance to determine whether each binder polymer is suitablefor slurry coating or not

The melting point (T_(m)) of the binder polymer (particularly, the firstbinder polymer and the second binder polymer) is determined by using adifferential scanning calorimeter (DSC) in a scanning range of −60 to200° C.

According to an embodiment of the present disclosure, PVdF-HFP as thefirst binder polymer may have vinylidene-derived repeating units andhexafluoropropylene-derived repeating units, and may includehexafluoropropylene-derived repeating units at a substitution ratio of15% or less, particularly at a substitution ratio of 10-15%, based onthe total repeating units (vinylidene-derived repeating units andhexafluoropropylene-derived repeating units).

In addition, PVdF-TFE as the second binder polymer may havevinylidene-derived repeating units and tetrafluoroethylene-derivedrepeating units, and may include tetrafluoroethylene-derived repeatingunits at a substitution ratio of 24% or less, particularly at asubstitution ratio of 20-24%, based on the total repeating units(vinylidene-derived repeating units and tetrafluoroethylene-derivedrepeating units).

Herein, ‘substitution ratio’ means the ratio (%) of the number ofrepeating units derived from a specific comonomer relative to the numberof total repeating units of a polymer.

The first binder polymer may have a weight average molecular weight(M_(w)) of 10,000-1,500,000. For example, the first binder polymer mayhave a weight average molecular weight (M_(w)) of 10,000-600,000, or100,000-600,000. When the first binder polymer has an excessively highweight average molecular weight, it shows low solubility and the bindersolution has excessively increased viscosity, thereby making itdifficult to carry out coating (application). When the first binderpolymer has an excessively low weight average molecular weight, it isdifficult to obtain a uniform coating layer.

The first binder polymer may be used in an amount of 5-50 parts byweight, or 10-35 part by weight, based on 100 parts by weight of theinorganic particles. When the first binder polymer is used in theabove-defined range, it is possible to provide an adequate level ofadhesion not only between the porous polymer substrate and the porouscoating layer but also between the porous coating layer and anelectrode.

The second binder polymer may have a weight average molecular weight(M_(w)) of 10,000-1,500,000. For example, the second binder polymer mayhave a weight average molecular weight (M_(w)) of 10,000-600,000, or100,000-600,000. When the second binder polymer has an excessively highweight average molecular weight, it shows low solubility and the bindersolution has excessively increased viscosity, thereby making itdifficult to carry out coating (application). When the second binderpolymer has an excessively low weight average molecular weight, it isdifficult to obtain a uniform coating layer.

The weight ratio of the first binder polymer to the second binderpolymer may be 50:50-85:15, 50:50-80:20, or 50:50-75:25. When the weightratio satisfies the above-defined range, it is possible to realizeviscosity suitable for application during coating. It is also possibleto prevent the problems caused by an excessively high or excessively lowelectrolyte uptake, thereby controlling phase separation with ease. As aresult, it is possible to form a porous coating layer structure andpores thereof having low resistance, high air permeability, high coatinglayer packing density and improved adhesion to an electrode.

According to an embodiment of the present disclosure, the inorganicparticles are not particularly limited, as long as they areelectrochemically stable. In other words, the inorganic particles arenot particularly limited, as long as they cause no oxidation and/orreduction in the operating voltage range (e.g. 0-5V based on Li/Li⁺) ofan applicable electrochemical device. For example, the inorganicparticles may have an average particle diameter of 0.001-3 μm, or0.001-2 μm. When the average particle diameter of the inorganicparticles satisfies the above-defined range, it is possible to improvedispersibility and to prevent an excessive increase in coating layer.Herein, the average particle diameter of the inorganic particles meansthe particle size (D₅₀) of 50% of the integrated value from a smallerparticle diameter calculated based on the results of measurement ofparticle size distribution of the particles after the classificationthereof using a conventional particle size distribution measuringsystem.

Non-limiting examples of the inorganic particles include Al₂O₃, A100H,Al(OH)₃, AlN, BN, MgO, Mg(OH)₂, SiO₂, ZnO, TiO₂, BaTiO₃, or a mixture ofat least two of them.

The content of the inorganic particles in the porous coating layer maybe 50-80 wt % based on 100 wt % of the porous coating layer.

According to an embodiment of the present disclosure, the porous coatinglayer includes inorganic particles and a binder polymer by which theinorganic particles are attached to one another to retain their bindingstate (i.e. the binder connects the inorganic particles with one anotherand fixes). In addition, the inorganic particles can retain theirbinding state to the porous polymer substrate by the binder. Herein, theporous coating layer has a porous structure on the surface thereofdifferent from the porous structure at the inner part thereof.

The surface of the porous coating layer includes a plurality of nodesincluding the inorganic particles and the binder polymer covering atleast a part of the surface of the inorganic particles, and at least onefilament formed from the binder polymer of the nodes in a thread-likeshape, wherein the filament may include a node-linking portion extendedfrom the node and connecting the node with another node.

The node-linking portion on the coating layer surface may include athree-dimensional network structure similar to a nonwoven structure,formed by the binder polymer-derived filaments crossing one another. Inthe network structure, the inorganic particles are at least partiallyembedded in the binder polymer filaments, and may be distributed in sucha manner that the inorganic particles may be spaced apart from oneanother at a specific interval by means of the filament(s). Preferably,the inorganic particles at the node-linking portion are spaced apartfrom one another by at least the average diameter of the inorganicparticles with a view to ensuring high porosity. According to anembodiment of the present disclosure, the filaments preferably have adiameter smaller than the diameter of the nodes.

According to an embodiment of the present disclosure, at the inner partof the porous coating layer, the plurality of nodes including theinorganic particles and the binder polymer covering at least a part ofthe surface of the inorganic particles may form interstitial volumes,while being in close contact with one another. Herein, the interstitialvolume means a space defined by the nodes facing each othersubstantially in a closely packed or densely packed structure of thenodes. The interstitial volumes among the nodes may be converted intovacant spaces to form pores. The pores at the inner part of the porouscoating layer totally may have a size smaller than the diameter of theinorganic particles.

As a result, the pores of the porous coating layer according to anembodiment of the present disclosure may have a unique porous structuredifferent from the other porous structures, such as a finger-likestructure formed by diffusion of the binder polymer in a non-solvent orthe Benard cell structure formed by partial opening of the binderpolymer at a portion including the binder polymer.

According to an embodiment of the present disclosure, the porous coatinglayer may have an average pore size of 10-400 nm, or 20-100 nm. The poresize may be calculated from the shape analysis through scanning electronmicroscopic (SEM) images, wherein the pore size is calculated by takinga closed curve formed by crossing of binder threads as a pore shape.According to an embodiment of the present disclosure, the pore size ofthe porous coating layer may be determined by capillary flow porometry.The capillary flow porometry is a method by which the smallest porediameter in the thickness direction is measured. Therefore, in order tomeasure only the pore size of the porous coating layer through capillaryflow porometry, it is required to separate the porous coating layer fromthe porous substrate and to measure the pore size, while the separatedporous coating layer is surrounded with a non-woven fabric capable ofsupporting the separated porous coating layer. Herein, the pore size ofthe non-woven fabric should be significantly larger than the pore sizeof the coating layer. According to an embodiment of the presentdisclosure, the porous coating layer preferably has a porosity of50-85%. When the porosity is 85% or less, it is possible to ensuredynamic property with which a pressing process for adhesion with anelectrode can be tolerated, and to prevent an excessive increase insurface opening, thereby facilitating adhesion. Meanwhile, when theporosity is 50% or more, it is possible to provide preferred ionpermeability, since it is higher than the porosity of the most of poroussubstrate.

Meanwhile, according an embodiment of the present disclosure, theporosity may be determined by using BELSORP (BET system) available fromBEL JAPAN Co., mercury intrusion porosimetry, capillary flowporosimetery, or the like. According to an embodiment of the presentdisclosure, the net density of an electrode active material layer iscalculated from the density (apparent density) of a finished electrode(electrode active material layer) and the compositional ratio ofingredients contained in the electrode (electrode active material layer)and density of each ingredient. Then, the porosity of an electrodeactive material layer may be calculated from the difference between theapparent density and the net density.

The porous coating layer may have a thickness of 0.5-5 μm on one side ofthe porous substrate preferably. The thickness may be 0.5 μm or more,preferably 1 μm or more. Within the above-defined range, it is possibleto obtain excellent adhesion with an electrode, thereby providingimproved cell strength of a battery. Meanwhile, when the thickness is 5μm or less, it is possible to provide a battery with preferred cyclecharacteristics and resistance characteristics. In this context, thethickness is preferably 4 μm or less, and more preferably 3 μm or less.

The separator for an electrochemical device according to an embodimentof the present disclosure may be obtained by the method comprising thesteps of:

(S1) preparing slurry for a porous coating layer comprising a solvent,inorganic particles and a binder polymer, wherein the binder polymercomprises a first binder polymer and a second binder polymer;

(S2) applying the slurry for a porous coating layer to at least onesurface of a porous substrate; and

(S3) dipping the resultant product of step (S2) in a solidifyingsolution including a non-solvent,

wherein the first binder polymer is poly(vinylidenefluoride-co-hexafluoroproyplene) (PVdF-HFP), and the second binderpolymer is poly(vinylidene fluoride-co-tetrafluoroethylene) (PVdF-TFE),

the first binder polymer has an electrolyte uptake of 80-165%,

the second binder polymer has an electrolyte uptake of 20-40%, and

the slurry for a porous coating layer has a viscosity value of 100-400cp, as determined at a shear rate of 100/s.

The method will be explained in more detail hereinafter.

First, a solvent, inorganic particles and a binder polymer are mixed toprepare slurry for a porous coating layer, wherein the binder polymerincludes the first binder polymer and the second binder polymer.

Next, a planar porous polymer substrate having a plurality of pores isprepared. The porous polymer substrate that may be used herein is thesame as described hereinabove.

The slurry for forming a porous coating layer is applied to at least onesurface of the porous polymer substrate. The slurry may be appliedthrough a conventional coating process, such as Meyer bar coating, diecoating, reverse roll coating, gravure coating, or the like. When theporous coating layers are formed on both surfaces of the poroussubstrate, the coating solution may be applied to one surface and thento the other surface, and then solidified, washed with water and dried.However, it is preferred in terms of productivity that the coatingsolution is applied to both surfaces at the same time, and thensolidified, washed with water and dried.

The inorganic particles and the binder polymer are the same as describedabove.

The solvent used for the slurry forming a porous coating layer has asolubility parameter similar to the solubility parameter of the binderpolymer to be used and has a low boiling point, preferably. This isbecause such a solvent allows homogeneous mixing and facilitates thesubsequent solvent removal.

The solvent may be used in an amount of 300-2,000 parts by weight, or500-1,500 parts by weight, based on 100 parts by weight of the combinedweight of the first binder polymer and the second binder polymer. It ispossible to obtain a binder solution having a viscosity of 30 centipoise(cp) to 500 cp from the above-defined range.

Non-limiting examples of the solvent include any one selected from thegroup consisting of acetone, methyl ethyl ketone, tetrahydrofuran,methylene chloride, chloroform, dimethylformamide,N-methyl-2-pyrrolidone (NMP) and cyclohexane, or a mixture of at leasttwo of them.

The slurry for a porous coating layer may have a viscosity of 100-400cp, as determined at a shear rate of 100/s. According to an embodimentof the present disclosure, the slurry for a porous coating layer mayhave a viscosity of 100-300 cp, or 100-290 cp. When the viscosity of theslurry for a porous coating layer satisfies the above-defined range, itis possible to apply the slurry uniformly to the porous substrate, andthus to provide a separator having improved stability by virtue ofconstant values of resistance and air permeability.

Next, the resultant product of the preceding step is dipped in asolidifying solution including a non-solvent.

The non-solvent means a solvent in which the binder polymer is notdissolved. The non-solvent is not particularly limited, as long as it ismiscible with the solvent used for facilitating phase separation.

Non-limiting examples of the non-solvent include water, methanol,ethanol, isopropanol, or a mixture of at least two of them.

In addition, in the step of dipping the product in a solidifyingsolution including the non-solvent, a solvent may be partially mixed tocontrol the phase separation rate, when the phase separation behavior isexcessively rapid. Herein, the amount of the solvent to be introduced ispreferably 30 vol % or less based on the total volume of thenon-solvent.

Since the non-solvent accelerates phase separation of the binder polymerin the slurry for forming a porous coating layer, it allows the binderpolymer to be present in a larger amount on the surface portion of theporous coating layer. Thus, the bindability of the separator to anelectrode is increased after the drying step as described below, therebyfacilitating lamination.

According to an embodiment of the present disclosure, the step ofdipping the product in a solidifying solution may be carried out bypreparing two or more solidifying solutions, and dipping the separatorcoated with the slurry sequentially in each of the solidifying solutionsfor a predetermined time. Herein, the solidifying solutions may beprepared in such a manner that the concentration of the non-solvent maybe increased sequentially as compared to the preceding step. Theconcentration of the non-solvent in at least the second or latersolidifying solution may be higher than the concentration of thenon-solvent in the first solidifying solution. For example, theconcentration of the non-solvent in the first solidifying solution maybe 95 wt % and that of the non-solvent in the later solidifying solutionmay be controlled to be higher than 95 wt %.

Since the solvent in the inorganic coating layer is exchanged with thesolidifying solution and the proportion of the non-solvent in thecoating layer is increased gradually, while the separator is dipped inthe solidifying solution including an excessive amount of non-solvent,it is preferred to gradually increase the proportion of the non-solventin a solidifying solution when the solidification is carried out througha plurality of steps by preparing a plurality of solidifying solutions.Meanwhile, when the non-solvent in the first solidifying solution is100%, the solidifying solutions after the first run include thenon-solvent alone.

According to an embodiment of the present disclosure, the solidifyingsolution may be maintained at a temperature equal to or higher than 5°C. and lower than 20° C. At a temperature lower than the above-definedrange, condensation of the non-solvent occurs undesirably. At atemperature higher than the above-defined range, phase separation occursrapidly so that the coating layer may not have a dense structure. Thus,an inorganic coating layer including desired nodes and filamentsaccording to the present disclosure cannot be formed and the separatorhas an excessively dense structure of the binder at a partial region,which is not preferred in terms of resistance characteristics andrealization of adhesion. Meanwhile, when a plurality of solidifyingsteps is carried out by preparing a plurality of solidifying solutionsas mentioned above, the first solidifying solution is set to atemperature equal to or higher than 5° C. and lower than 20° C., andthen the temperature of the second or later solidifying solution may beincreased sequentially until the drying step is carried out. At least,the second or the later solidifying solution may be prepared to have atemperature higher than the temperature of the first solidifyingsolution. However, it is preferred to control the temperature of thesecond or later solidifying solution to a temperature equal to 40° C. orlower. At a temperature higher than the above-defined range, evaporationof the non-solvent occurs undesirably. At a temperature lower than theabove-defined range, thermal impact occurs upon the introduction to adrying furnace, resulting in a risk of a change in width of thesubstrate.

Meanwhile, according to an embodiment of the present disclosure, thedipping tine may be controlled to 30-100 seconds, or 40-90 seconds. Whenthe dipping time satisfies the above-defined range, it is possible tosolve the problem of degradation of adhesion between the poroussubstrate and the porous coating layer, caused by excessive phaseseparation, and separation of the coating layer, and to form uniformpores in the thickness direction. Meanwhile, when a plurality ofsolidifying steps is carried out by preparing a plurality of solidifyingsolutions as mentioned above, the dipping time in the first solidifyingsolution may be controlled to 10-30 seconds.

Then, the resultant product obtained after dipping the separator in thesolidifying solution may be dried to form a porous coating layer on atleast one surface of the porous substrate. The drying step may becarried out in a drying chamber and the drying chamber is not limited toany particular condition due to the application of the non-solvent. Inthis manner, the porous coating layer includes a larger amount of thebinder polymer on the surface portion thereof, as compared to the lowerpart thereof. Thus, it is possible to provide improved binding force toan electrode as mentioned above. In addition, since the porous coatinglayer is formed to the same height as a whole, although the portionrealizing adhesion to an electrode is present, the separator has thesame resistance in the surface direction.

Then, the separator obtained by the above-described method may beinterposed between a positive electrode and a negative electrode andlaminated therewith to provide an electrochemical device. Theelectrochemical device includes any device which carries outelectrochemical reaction, and particular examples thereof include alltypes of primary batteries, secondary batteries, fuel cells, solar cellsor capacitors, such as super capacitor devices. Particularly, among thesecondary batteries, lithium secondary batteries, including lithiummetal secondary batteries, lithium ion secondary batteries, lithiumpolymer secondary batteries or lithium ion polymer secondary batteries,are preferred.

The positive electrode and the negative electrode used in combinationwith the separator according to the present disclosure are notparticularly limited, and may be obtained by allowing electrode activematerials to be bound to at least one surface of an electrode currentcollector through a method generally known in the art.

Among the electrode active materials, non-limiting examples of apositive electrode active material include conventional positiveelectrode active materials that may be used for the positive electrodesfor conventional electrochemical devices. Particularly, lithiummanganese oxides, lithium cobalt oxides, lithium nickel oxides, lithiumiron oxides or lithium composite oxides containing a combination thereofare used preferably.

Non-limiting examples of a negative electrode active material includeconventional negative electrode active materials that may be used forthe negative electrodes for conventional electrochemical devices.Particularly, lithium-intercalating materials, such as lithium metal orlithium alloys, carbon, petroleum coke, activated carbon, graphite orother carbonaceous materials, are used preferably.

Non-limiting examples of a positive electrode current collector includefoil made of aluminum, nickel or a combination thereof. Non-limitingexamples of a negative electrode current collector include foil made ofcopper, gold, nickel, or copper alloys or a combination thereof.

The electrolyte that may be used in the electrochemical device accordingto the present disclosure is a salt having a structure of A⁺B⁻, whereinA⁺ includes an alkali metal cation such as Li⁺, Na⁺, K⁺ or a combinationthereof, and B⁻ includes an anion such as PF₆ ⁻, BF₄ ⁻, Cl⁻, Br⁻, I⁻,ClO₄ ⁻, AsF₆ ⁻, CH₃CO₂ ⁻, CF₃SO₃ ⁻, N(CF₃SO₂)₂ ⁻, C(CF₂SO₂)₃ ⁻ or acombination thereof, the salt being dissolved or dissociated in anorganic solvent including propylene carbonate (PC), ethylene carbonate(EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropylcarbonate (DPC), dimethyl sulfoxide, acetonitrile, dimethoxyethane,diethoxyethane, tetrahydrofuran, N-methyl-2-pyrrolidone (NMP), ethylmethyl carbonate (EMC), gamma-butyrolactone (γ-butyrolactone) or amixture thereof. However, the present disclosure is not limited thereto.

Injection of the electrolyte may be carried out in an adequate stepduring the process for manufacturing a battery depending on themanufacturing process of a final product and properties required for afinal product. In other words, injection of the electrolyte may becarried out before the assemblage of a battery or in the final step ofthe assemblage of a battery.

Examples will be described more fully hereinafter so that the presentdisclosure can be understood with ease. The following examples may,however, be embodied in many different forms and should not be construedas limited to the exemplary embodiments set forth therein. Rather, theseexemplary embodiments are provided so that the present disclosure willbe thorough and complete, and will fully convey the scope of the presentdisclosure to those skilled in the art.

Example 1

Manufacture of Separator

First, 28 parts by weight of polyvinylidenefluoride-co-hexafluoropropylene (PVdF-HFP) (Solef 21510 available fromSolvay Co., T_(m) (melting point) 132° C., electrolyte uptake 110%, HFPsubstitution ratio 15%) as the first binder polymer, and 7 parts byweight of (polyvinylidene fluoride-co-tetrafluoroethylene) (PVdF-TFE)(VT-475 available from Daikin Co., T_(m) 138° C., electrolyte uptake30%, TFE substitution ratio 24%) as the second binder polymer were addedto 400 parts by weight of N-methyl-2-pyrrolidone (NMP) and dissolvedtherein at 50° C. for about 12 hours or more to obtain a bindersolution.

Herein, the first binder polymer had a viscosity of 60 cp, and thesecond binder polymer had a viscosity of 700 cp. Herein, each of theviscosity of the first binder polymer and that of the second binderpolymer is the viscosity value determined from slurry containing 35parts by weight of the corresponding binder polymer, 65 parts by weightof Al₂O₃ having an average particle diameter (D₅₀) of 500 nm (LS235available from Japanese Light Metal Co.), and 400 parts by weight ofN-methyl-2-pyrrolidone (NMP) by using a rheometer (TA instruments), at ashear rate of 100/s.

Then, 65 parts by weight of Al₂O₃ having an average particle diameter(D₅₀) of 500 nm (LS235 available from Japanese Light Metal Co.), asinorganic particles, were added to the binder solution and dispersedtherein by using a paint shaker to prepare slurry for forming a porouscoating layer.

Herein, the slurry for forming a porous coating layer had a viscosity of110 cp, as determined at a shear rate of 100/s by using a rheometer (TAinstruments).

After that, the resultant slurry for forming a porous coating layer wascoated on a polyethylene porous polymer substrate having a thickness of12 μm (porosity 45%) through a rod coating process, and then was dippedin water as a non-solvent to carry out phase separation. The dippingtime was 40 seconds. Then, the resultant product was dried in an oven ata temperature of 75° C. to obtain a separator for an electrochemicaldevice.

Manufacture of Positive Electrode and Negative Electrode

First, 97 wt % of LiCoO₂, 1.5 wt % of carbon black powder as aconductive material and 1.5 wt % of polyvinylidene fluoride (PVdF,Kureha) were mixed and introduced to N-methyl-2-pyrrolidone as asolvent. Then, the resultant mixture was agitated by using a mechanicalagitator for 30 minutes to prepare positive electrode active materialslurry. The slurry was applied to an aluminum current collector having athickness of 20 μm by using a doctor blade to a thickness of about 60μm, dried in a hot air dryer at 100° C. for 0.5 hours, and then furtherdried at 120° C. for 4 hours, followed by roll pressing, to obtain apositive electrode.

Meanwhile, 96.5 wt % of artificial graphite particles (LC1, Shanshan)having an average particle diameter of 16 μm, 2.3 wt % ofstyrene-butadiene rubber (SBR) binder (ZEON) and 1.2 wt % ofcarboxymethyl cellulose (CMC, Daicel) were mixed and introduced todistilled water. The resultant mixture was agitated by using amechanical agitator for 60 minutes to obtain negative electrode activematerial slurry. The slurry was applied to a copper current collectorhaving a thickness of 8 μm by using a doctor blade to a thickness ofabout 60 μm, dried in a hot air dryer at 100° C. for 0.5 hours, and thenfurther dried at 120° C. for 4 hours, followed by roll pressing, toobtain a negative electrode.

Manufacture of Lithium Secondary Battery

A separator was interposed between the positive electrode and thenegative electrode obtained as described above, and lamination wascarried out at 80° C. under a load of 100 kgf to obtain a unit cell. Theunit cell was inserted into a pouch, and an organic electrolytecontaining 1M LiPF₆ dissolved in a mixed solvent of ethylene carbonate(EC)/ethyl methyl carbonate (EMC)/diethyl carbonate (DEC) at a ratio of3/5/2 (volume ratio) was injected thereto. Then, the pouch wasvacuum-sealed and activation was carried out to finish a lithiumsecondary battery.

Example 2

A separator and a lithium secondary battery were obtained in the samemanner as Example 1, except that PVdF-HFP (Solef 21510) was used in anamount of 17.5 parts by weight, PVdF-TFE (VT-475) was used in an amountof 17.5 parts by weight, the slurry for a porous coating layer had aviscosity of 209 cp at a shear rate of 100/s, the slurry for forming aporous coating layer was coated on a polyethylene porous polymersubstrate (porosity 45%) having a thickness of 12 μm through a rodcoating process, and the dipping time was 40 seconds when the phaseseparation is carried out by dipping the separator in water as anon-solvent.

Example 3

A separator and a lithium secondary battery were obtained in the samemanner as Example 1, except that the dipping time in water as anon-solvent was 90 seconds.

Example 4

A separator and a lithium secondary battery were obtained in the samemanner as Example 1, except that Kynar 2500 available from Arkema Co.was used as PVdF-HFP.

Example 5

A separator and a lithium secondary battery were obtained in the samemanner as Example 1, except that Kynar 2751 available from Arkema Co.was used as PVdF-HFP.

Example 6

A separator and a lithium secondary battery were obtained in the samemanner as Example 2, except that Kynar 2500 available from Arkema Co.was used as PVdF-HFP.

Example 7

A separator and a lithium secondary battery were obtained in the samemanner as Example 2, except that Kynar 2751 available from Arkema Co.was used as PVdF-HFP.

Comparative Example 1

A separator and a lithium secondary battery were obtained in the samemanner as Example 1, except that 35 parts by weight of PVdF-HFP (Solef21510) was used alone as a binder polymer, 65 parts by weight of Al₂O₃was used, and the slurry for a porous coating layer had a viscosity of60 cp at a shear rate of 100/s.

Comparative Example 2

A separator and a lithium secondary battery were obtained in the samemanner as Example 1, except that 70 parts by weight of PVdF-HFP (Solef21510) was used alone as a binder polymer, 30 parts by weight of Al₂O₃was used, and the slurry for a porous coating layer had a viscosity of75 cp at a shear rate of 100/s.

Comparative Example 3

A separator and a lithium secondary battery were obtained in the samemanner as Comparative Example 1, except that the dipping time in wateras a non-solvent was 90 seconds.

Comparative Example 4

A separator and a lithium secondary battery were obtained in the samemanner as Example 1, except that 8200 available from Kureha Corp. wasused as PVdF-HFP.

Comparative Example 5

A separator and a lithium secondary battery were obtained in the samemanner as Example 1, except that LB G available from Arkema Co. was usedas PVdF-HFP.

Comparative Example 6

A separator and a lithium secondary battery were obtained in the samemanner as Example 1, except that the second binder polymer wasintroduced in the same amount as the first binder polymer, instead ofthe first binder polymer.

Comparative Example 7

A separator and a lithium secondary battery were obtained in the samemanner as Example 1, except that PVdF-CTFE (Solef 32008 available fromSolvay Co.) was used in the same amount as PVdF-TFE as the second binderpolymer, instead of PVdF-TFE.

Comparative Example 8

A separator and a lithium secondary battery were obtained in the samemanner as Example 1, except that PVdF-CTFE (Solef 32008 available fromSolvay Co.) was used in the same amount as PVdF-HFP as the first binderpolymer, instead of PVdF-HFP.

Comparative Example 9

A separator and a lithium secondary battery were obtained in the samemanner as Example 1, except that PVdF homopolymer (1100 available fromKureha Corp.) was used in the same amount as PVdF-TFE as the secondbinder polymer, instead of PVdF-TFE.

Test Examples

Observation of Cross Section of Separator

The section of each of the separators according to Example 1 andComparative Example 1 was observed by using a scanning electronmicroscope (FE-SEM) (Hitachi S-4800 Scanning Electron Microscope). TheSEM image of the cross section of the separator according to Example 1is shown in FIG. 1 and the SEM image of the cross section of theseparator according to Comparative Example 1 is shown in FIG. 2.

Referring to FIG. 1, it can be seen that the inner part of the porouscoating layer of the separator according to Example 1 has a portion inwhich a plurality of nodes including the inorganic particles and thebinder polymer covering at least a part of the surface of the inorganicparticles are concentrated, wherein the nodes form interstitial volumeswhile being in close contact with one another, and the pores formed bythe interstitial volumes converted into vacant spaces totally have asize smaller than the diameter of the inorganic particles. On thecontrary, referring to FIG. 2, it can be seen that the porous coatinglayer of the separator according to Comparative Example 1 has a porousstructure in which a plurality of finger-like structured pores having asignificantly large size are present due to the use of PVdF-HFP having ahigh phase separation rate alone as a binder polymer and rapid diffusionof the binder polymer into the non-solvent.

Method for Determining Electrolyte Uptake of Binder Polymer (FirstBinder Polymer and Second Binder Polymer)

A film was prepared by using the binder polymer through a phaseseparation process, was cut into a circular shaped film (diameter 1.8cm; area 2.54 cm²; thickness 16 μm (deviation, −0.5 to ˜+0.5 μm)), andthen was weighed. Particularly, the film was obtained through dippingphase separation of a solution in which each binder polymer wasdissolved by using a non-solvent. Next, the circular shaped binderpolymer film was introduced to a mixed electrolyte solution (EC/DEC=1/1or EC/PC=1/1 volume ratio) containing 1.0M of lithium salt (LiPF₆)dissolved therein and was removed therefrom after 24 hours. Then, thebinder polymer film was allowed to stand in its vertically standingstate for 1 minute to remove the liquid droplets on the film surface.The resultant film impregnated with the mixed electrolyte solution wasweighed. The whole test was carried out in a glove box filled with argongas under a condition containing water less than 1.0 ppm. Theelectrolyte uptake (%) was calculated by the formula of [(Weight of filmafter uptake−Weight of film before uptake)/(Weight of film beforeuptake)]×100.

Method for Determining Resistance

The resistance of each of the separators according to Examples 1-7 andComparative Examples 1-9 was determined as follows. An electrolyte wasprepared by dissolving 1M LiPF₆ in a mixed solvent containing ethylenecarbonate, propylene carbonate and propyl propionate at a ratio (volumeratio) of 25:10:65. After each separator was allowed to uptake theelectrolyte, a coin cell was manufactured and the resistance of eachcoin cell was determined by using an electrochemical impedancespectroscopy (EIS) system. The results are shown in the following Table1.

Method for Determining Air Permeability

The air permeability of each of the separators according to Examples 1-7and Comparative Examples 1-9 was determined by using a Gurley type airpermeability tester according to JIS P-8117.

Herein, the time required for 100 mL of air to pass through a diameterof 28.6 mm and an area of 645 mm² was measured. The results are shown inthe following Table 1.

Adhesion to Substrate (Peel Force)

Each of the separators according to Examples 1-7 and ComparativeExamples 1-9 was cut into a size of 100 mm (length)×25 mm (width) toprepare two test specimens. The two test specimens of each of theseparators according to Examples 1-7 and Comparative Examples 1-9 wasstacked and subjected to hot pressing at 100° C. for 10 seconds toobtain a laminate. The laminate was fixed to an adhesion strength tester(LLOYD Instrument, LF plus) and the upper separator specimen was peeledoff at 25° C. and a rate of 100 mm/min with an angle of 180°, and theadhesion strength was measured. The results are shown in the followingTable 1.

Method for Determining Adhesion between Separator and Electrode(Adhesion to Electrode, Lami Strength)

The negative electrode according to Example 1 was cut into a size of 25mm×100 mm. Each of the separators according to Examples 1-7 andComparative Examples 1-9 was cut into a size of 25 mm×100 mm. Eachseparator was stacked with the negative electrode, inserted between PETfilms (100 μm) and adhered with each other by using a flat press.Herein, the flat press carried out heating at 70° C. under a pressure of600 kgf (3.9 MPa) for 1 second. The end of the adhered separator andnegative electrode was mounted to an UTM system (LLOYD Instrument LFPlus), and then the force required for separating the negative electrodefrom the porous coating layer facing the negative electrode was measuredby applying force thereto at 180° with a rate of 300 mm/min.

TABLE 1 Binder Adhesion to polymer Slurry Dipping Air substrate Adhesionto composition First binder Second binder viscosity time Resistancepermeability (peel force) electrode (weight ratio) polymer polymer (cp)(seconds) (ohm) (s/100 cc) (gf/25 mm) (gf/25 mm) Ex. 1 PVDF-HFP:PVDF-HFP PVdF-TFE 110 40 0.98 130 156 177 PVdF-TFE = ElectrolyteElectrolyte 4:1 uptake 110% uptake 30% Viscosity Viscosity 60 cP 700 cPEx. 2 PVDF-HFP: PVDF-HFP PVdF-TFE 290 40 0.81 135 285 191 PVdF-TFE =Electrolyte Electrolyte 1:1 uptake 110% uptake 30% Viscosity Viscosity60 cP 700 cP Ex. 3 PVDF-HFP: PVDF-HFP PVdF-TFE 110 90 0.96 132 150 168PVdF-TFE = Electrolyte Electrolyte 4:1 uptake 110% uptake 30% ViscosityViscosity 60 cP 700 cP Ex. 4 PVDF-HFP: PVDF-HFP PVdF-TFE 100 40 1.01 133177 182 PVdF-TFE = Electrolyte Electrolyte 4:1 uptake 120% uptake 30%Viscosity Viscosity 45 cP 700 cP Ex. 5 PVDF-HFP: PVDF-HFP PVdF-TFE 12040 0.94 120 137 191 PVdF-TFE = Electrolyte Electrolyte 4:1 uptake 100%uptake 30% Viscosity Viscosity 70 cP 700 cP Ex. 6 PVDF-HFP: PVDF-HFPPVdF-TFE 260 40 0.85 135 291 187 PVdF-TFE = Electrolyte Electrolyte 1:1uptake 120% uptake 30% Viscosity Viscosity 45 cP 700 cP Ex. 7 PVDF-HFP:PVDF-HFP PVdF-TFE 300 40 0.77 132 276 203 PVdF-TFE = ElectrolyteElectrolyte 1:1 uptake 100% uptake 30% Viscosity Viscosity 70 cP 700 cPComp. PVDF-HFP: PVDF-HFP 60 40 1.15 140 125 130 Ex. 1 PVdF-TFE =Electrolyte 1:0 uptake 110% Viscosity 60 cP Comp. PVDF-HFP: PVDF-HFP 7540 1.27 145 115 120 Ex. 2 PVdF-TFE = Electrolyte 1:0 uptake 110%Viscosity 60 cP Comp. PVDF-HFP: PVDF-HFP 60 90 1.22 155 20 50 Ex. 3PVdF-TFE = Electrolyte 1:0 uptake 110% Viscosity 60 cP Comp. PVDF-HFP:PVDF-HFP PVdF-TFE 350 40 0.90 125 133 72 Ex. 4 PVdF-TFE = ElectrolyteElectrolyte 4:1 uptake 60% uptake 30% Viscosity Viscosity 220 cP 700 cPComp. PVDF-HFP: PVDF-HFP PVdF-TFE 330 40 0.91 129 164 91 Ex. 5 PVdF-TFE= Electrolyte Electrolyte 4:1 uptake 70% uptake 30% Viscosity Viscosity180 cP 700 cP Comp. PVDF-HFP: PVdF-TFE 650 40 0.88 131 311 21 Ex. 6PVdF-TFE = Electrolyte 0:1 uptake 30% Viscosity 700 cP Comp. PVDF-HFP:PVDF-HFP PVdF-CTFE 70 40 1.22 142 155 180 Ex. 7 PVdF-CTFE = ElectrolyteElectrolyte 4:1 uptake 110% uptake 210% Viscosity Viscosity 60 cP 90 PComp. PVDF-CTFE: PVdF-CTFE PVdF-TFE 130 40 1.18 138 217 33 Ex. 8PVdF-TFE = Electrolyte Electrolyte 4:1 uptake 210% uptake 30% ViscosityViscosity 90 cP 700 cP Comp. PVDF-HFP: PVDF-HFP PVdF 100 40 0.95 132 15297 Ex. 9 PVdF = Electrolyte Electrolyte 4:1 uptake 110% uptake 15%Viscosity Viscosity 60 cP 300 cP

Referring to Table 1, it can be seen that each of the separatorsaccording to Example 1-7, which uses the first binder polymer having anelectrolyte uptake of 80-165% and the second binder polymer having anelectrolyte uptake of 20-40%, shows low resistance, high airpermeability, and improved adhesion to an electrode and adhesion to asubstrate.

On the contrary, when using the first binder polymer alone (ComparativeExamples 1-3), resistance is increased and peel force and adhesion to anelectrode are significantly reduced. In addition, when using the secondbinder polymer alone (Comparative Example 6), adhesion to an electrodeis significantly reduced. Further, even though two types of binderpolymers having a different electrolyte uptake are used as the firstbinder polymer and the second binder polymer, adhesion to an electrodeis significantly reduced or resistance is increased, when each binderpolymers has an electrolyte uptake not within the above-defined range ofelectrolyte uptake, i.e. an electrolyte uptake of 80-165% in the case ofthe first binder polymer, and an electrolyte uptake of 20-40% in thecase of the second binder polymer (Comparative Examples 4, 5 and 7-9).

1. A separator for an electrochemical device, comprising: a poroussubstrate having a plurality of pores; and a porous coating layerpositioned on at least one surface of the porous substrate, the porouscoating layer comprising a plurality of inorganic particles and a binderpolymer positioned on a whole or a part of a surface of the inorganicparticles to connect the inorganic particles with one another and fixthe inorganic particles, wherein the binder polymer comprises a firstbinder polymer and a second binder polymer, the first binder polymer ispoly(vinylidene fluoride-co-hexafluoroproyplene) (PVdF-HFP), and thesecond binder polymer is poly(vinylidenefluoride-co-tetrafluoroethylene) (PVdF-TFE), the first binder polymerhas an electrolyte uptake of 80-165%, and the second binder polymer hasan electrolyte uptake of 20-40%.
 2. The separator for theelectrochemical device according to claim 1, wherein the first binderpolymer has a melting point of 130-135° C., and the second binderpolymer has a melting point of 135-140° C.
 3. The separator for theelectrochemical device according to claim 1, wherein the first binderpolymer has a viscosity of 50-70 cp, the second binder polymer has aviscosity of 600-800 cp, and the viscosity is determined at a shear rateof 100/s from slurry comprising 35 parts by weight of the first binderpolymer or the second binder polymer, 65 parts by weight of Al₂O₃ havingan average particle diameter (D₅₀) of 500 nm, and 400 parts by weight ofN-methyl-2-pyrrolidone (NMP).
 4. The separator for the electrochemicaldevice according to claim 1, wherein the first binder polymer has anelectrolyte uptake of 90-130%, and the second binder polymer has anelectrolyte uptake of 25-35%.
 5. The separator for the electrochemicaldevice according to claim 1, wherein a weight ratio of the first binderpolymer to the second binder polymer is 50:50-85:15.
 6. The separatorfor the electrochemical device according to claim 1, wherein the surfaceof the porous coating layer comprises a plurality of nodes including theinorganic particles and the binder polymer covering at least a part ofthe surface of the inorganic particles, and at least one filament formedfrom the binder polymer of the nodes in a thread-like shape, and thefilament comprises a node-linking portion extended from the node andconnecting the node with another node.
 7. The separator for theelectrochemical device according to claim 6, wherein the plurality ofnodes including the inorganic particles and the binder polymer coveringat least a part of the surface of the inorganic particles forminterstitial volumes, while being in close contact with one another,wherein at an inner part of the porous coating layer, the interstitialvolumes are converted into vacant spaces to form pores having a sizesmaller than a diameter of the inorganic particles.
 8. A method formanufacturing a separator for an electrochemical device, comprising:(S1) preparing slurry for forming a porous coating layer comprising asolvent, inorganic particles and a binder polymer, wherein the binderpolymer comprises a first binder polymer and a second binder polymer;(S2) applying the slurry for forming the porous coating layer to atleast one surface of a porous substrate; and (S3) dipping a resultantproduct of (S2) in a solidifying solution including a non-solvent,wherein the first binder polymer is poly(vinylidenefluoride-co-hexafluoroproyplene) (PVdF-HFP), and the second binderpolymer is poly(vinylidene fluoride-co-tetrafluoroethylenel (PVdF-TFE),the first binder polymer has an electrolyte uptake of 80-165%, thesecond binder polymer has an electrolyte uptake of 20-40%, and theslurry for forming the porous coating layer has a viscosity value of100-400 cp, as determined at a shear rate of 100/s.
 9. The method formanufacturing the separator for the electrochemical device according toclaim 8, wherein the first binder polymer has a melting point of130-135° C., and the second binder polymer has a melting point of135-140° C.
 10. The method for manufacturing the separator for theelectrochemical device according to claim 8, wherein the first binderpolymer has a viscosity of 50-70 cp, the second binder polymer has aviscosity of 600-800 cp, and the viscosity is determined at a shear rateof 100/s from slurry comprising 35 parts by weight of the first binderpolymer or the second binder polymer, 65 parts by weight of Al₂O₃ havingan average particle diameter (D₅₀) of 500 nm, and 400 parts by weight ofN-methyl-2-pyrrolidone (NMP).
 11. The method for manufacturing theseparator for the electrochemical device according to claim 8, whereinthe first binder polymer has an electrolyte uptake of 90-130%, and thesecond binder polymer has an electrolyte uptake of 25-35%.
 12. Themethod for manufacturing the separator for the electrochemical deviceaccording to claim 8, wherein a weight ratio of the first binder polymerto the second binder polymer is 50:50-85:15.
 13. An electrochemicaldevice comprising a positive electrode, a negative electrode and aseparator interposed between the positive electrode and the negativeelectrode, wherein the separator is the separator of claim
 1. 14. Theelectrochemical device according to claim 13, wherein theelectrochemical device is a secondary battery.